The present invention relates to a high-purity lanthanum, a method for producing high-purity lanthanum, a sputtering target formed from high-purity lanthanum, and a metal gate film having high-purity lanthanum as main component.
Lanthanum (La) is one of rare earth elements that exist in the form of mixed complex oxides as mineral resources in earth's crust. Rare earth elements were named as such since they were originally isolated from relatively rare minerals. However, their existence is not so rare if whole of earth's crust is taken into account.
The atomic number of Lanthanum is 57. It is a silvery white metal with atomic weight of 138.9 and has a multi hexagonal close-packed structure at ambient temperature. It has the melting point of 921° C., boiling point of 3500° C., and density of 6.15 g/cm3, and its surface is oxidized in air. It melts slowly in water, and is soluble in hot water as well as in acid. It is not ductile but exhibits slight malleability. Its specific resistance is 5.70×10−6 Ωcm. It combusts at 445° C. and above and forms an oxide (La2O3)(see Encyclopedia of Physical Chemistry).
Rare earth elements in general are stable as compounds with oxidation number of three, and lanthanum is also trivalent. Recently, a lot of research and development have focused on lanthanum as electronic material such as metal gate material and high dielectric constant material (High-k), making it one of the metals that is drawing a lot of attention.
Metal lanthanum has the problem of being readily oxidized during the purification process, and as such, it is a difficult material to work with in a highly purified form. For this reason, no highly purified product of lanthanum has been made available to date. In addition, metal lanthanum turns black by oxidation in a short period of time when left exposed to air, creating additional problem for handling.
In the next generation MOSFET, gate insulator needs to become even thinner than it currently is. SiO2, which has been traditionally used as gate insulator, however, is approaching its limits in usefulness in that it is increasingly becoming difficult to function properly at the required thinness, because of the increase in the leak current due to tunnel effect.
For this reason, HfO2, ZrO2, Al2O3 and La2O3 having high dielectric constant, high thermal stability and high energy barrier against electron holes and electrons in silicon, have been proposed as its potential alternatives. Among these materials, La2O3 is considered to be especially promising, and as such, its electrical characteristics have been studied, and its potential as gate insulator in the next generation MOSFET has been reported (see non-patent document 1). However, in this particular non-patent document, the subject of the study is limited to La2O3 film, and the characteristics and behavior of lanthanum element are not explored.
On the other hand, a technology in which halogenated rare earth metals are reduced by calcium or hydrogenated calcium was proposed about 2 decades ago as a method for isolating rare earth metals. This document listed lanthanum as an example of rare earths. However, the technology was a rudimentary one involving slag separating jig as a means of separating slag, and did not particularly disclose much about the problems associated with the use of metal lanthanum element as well as the method for its purification (see Patent Document 1).
As discussed above, the use of lanthanum (lanthanum oxide) is still in its early days and more research is required. In studying the property of lanthanum (lanthanum oxide), having a metal lanthanum itself as a sputtering target material would be highly beneficial because it would enable the formation of lanthanum thin film on a substrate and facilitate the research into the behavior of its interface with the silicon substrate as well as the properties of high dielectric constant gate insulator and the like made from lanthanum compounds produced. In addition, it would also greatly enhance the freedom of its use in various final products.
However, the problem of oxidation that can occur rapidly (in about 10 minutes) when exposed to air would persist even if such a lanthanum sputtering target is produced. Once the oxidized film is formed on the target, it would result in the reduction of electric conductivity and lead to defects in sputtering. Moreover, if the target is left exposed to air for a long period of time, it would react with the moisture in the air and can become covered with white hydroxide powder, which in turn makes sputtering impossible.
For this reason, measures for preventing oxidation, such as packing in vacuum and covering with oil, need to be taken immediately after the production of target. However, these are extremely cumbersome processes. Due to these problems, the target material using lanthanum element still has not been realized. As Patent Documents, there are three listed below (Patent Document 2 to Patent Document 5), by the same applicants of the present application.
Furthermore, generation of nodules on the surface of the target poses another problem when forming a film by sputtering with lanthanum target. These nodules elicit abnormal discharge, generating particles from the eruption of the nodules and the like.
Generation of particles in turn can increase the defect rate of metal gate films, semi-conductor elements and devices. Especially problematic is the presence of carbon (graphite), which is a solid. Since graphite is conductive, it is difficult to be detected. Thus improvement needs to be made to reduce its presence.
Although lanthanum, as discussed earlier, is a material hard to prepare in highly purified form, it is preferable to reduce the content of Al, Fe and Cu in addition to carbon (graphite) mentioned above, in order to take full advantage of the property of lanthanum. Furthermore, the presence of alkaline metals, alkali earth metals, transition metal elements, high melting point metal elements, and radioactive elements all adversely affect the property of semi-conductor and therefore need to be reduced. From these considerations, the purity of lanthanum is preferably 5N or more.
In the Patent Document 5 below, there is a disclosure about reducing the contents of Al, Fe and Cu to 100 wtppm, respectively, by acid washing and ultrasonic cleaning the lanthanum raw material, followed by electron beam melting. The Example 2 therein achieved Al content of 5.5 wtppm, Fe content of 3.5 wtppm and Cu content of 2.8 wtppm. Patent Document 5 succeeded in significantly reducing the contents of these elements and represented a considerable step forward. However, further improvement in the purity was needed, and the method for achieving that goal had to be researched and developed.
However, a problem exists in the extreme difficulty of removing rare earths, particularly lanthanoids other than lanthanum. Fortunately, minor contamination of lanthanoids other than lanthanum poses no major issues since their properties are similar enough to that of lanthanum. There are also materials containing less amounts of rare earths that can be used as the raw material. If further reduction of rare earths is required in particular, these materials can be used as the starting material. Likewise, minor contamination of gas components also poses no major problems. Gas component is generally very difficult to remove, and it is customary not to include the contribution from the gas component when indicating the purity.
Topics such as the physical property of lanthanum, production method for highly purified lanthanum, behavior of impurities in lanthanum target, have not been extensively explored to date. Therefore, it is highly desirable that these problems are adequately addressed as soon as possible.    Patent Document 1: Japanese Unexamined Patent Application Publication No. S63-11628    Patent Document 2: Japanese Patent Application No. 2009-547950    Patent Document 3: Japanese Patent Application No. 2009-078836    Patent Document 4: Japanese Patent Application No. 2009-084078    Patent Document 5: PCT International Publication No. WO2009/084318    Non-Patent Document 1: Eisuke Tokumitsu et. al. “Study of oxide materials for High-k gate insulator”. Research material for The Institute of Electrical Engineers of Japan, Committee on Electronic Materials. Vol. 6-13, page 37-41. Sep. 21, 2001.